1. Field of the Invention
The present invention relates to an optical recording and/or reproducing apparatus, and more particularly, to an optical pickup capable of preventing a tracking offset from being generated due to a shift of an objective lens and an initial balance deviation of a photodetector and an optical recording and/or reproducing apparatus adopting the optical pickup.
2. Description of Related Art
In high capacity recording and/or reproduction, accurate detection of a focusing and/or tracking error signal is necessary for performing a stable servo function. Generally, an optical pickup is composed of a light source, an objective lens, and a light-receiving optical system. The objective lens focuses light emitted from the light source on a recording surface of an optical disc, and the light-receiving optical system detects an information signal and an error signal from light reflected by the optical disc and passed through the objective lens.
U.S. Patent Publication No. 2002-0159378 A1 (published on Oct. 31, 2002) to the applicant of the present invention discloses an optical pickup capable of detecting a tracking error signal in which an offset generation due to a shift of an objective lens is low and a focusing error signal in which an offset generation due to a distortion of a photodetector or a temperature change (including a change of a wavelength of light) is low.
FIGS. 1 through 3 illustrate a diffraction unit 30, a photodetector 50, and a signal processing unit, respectively, which are disclosed in U.S. Patent Publication No. 2002-0559378 A1. The photodetector 50 receives light reflected by an optical disc and passed through the diffraction unit 30 of FIG. 1, and the signal processing unit detects a tracking error signal. Since a detailed description of FIGS. 1 through 3 has been made in the Detailed Description of the Invention of the above publication, only necessary parts will now be described.
Referring to FIG. 1, the diffraction unit 30 is divided into first through fifth diffraction areas E″, A″, B″, C″, and D″.
To transmit light with a specific wavelength travelling toward a recording medium and diffract light with the specific wavelength reflected by the recording medium, the diffraction unit 30 includes a polarization hologram layer (not shown) and a polarization changing layer (not shown) (i.e., a quarter wave plate) formed on a surface of the polarization hologram layer that faces the recording medium. To compatibly adopt CDs and DVDs, the diffraction unit 30 also includes an aperture filter (not shown) and a phase compensator (not shown). The aperture filter adjusts numerical apertures of the light with the specific wavelength and lights with other wavelengths. The phase compensator compensates for a spherical aberration generated upon data recording and/or reproduction from an optical disc having a thickness deviating from a design condition of an objective lens.
Referring to FIG. 2, the photodetector 50 includes first through fifth light-receiving portions 53, 55, 56, 57, and 58, which respectively receive lights with a specific wavelength reflected by the recording medium and diffracted by the first through fifth diffraction areas E″, A″, B″, C″, and D″ of the diffraction unit 30 and perform photoelectric conversion on the received lights. The photodetector 50 further includes a main light-receiving portion 51, which receives zeroth-order-diffracted light passed through the first through fifth diffraction areas E″, A″, B″, C″, and D″ and detects a reproduction signal from the received light.
The first light-receiving portion 53 is divided into a four-sectioned light-receiving area E, F, G, and H in radial and tangential directions of the optical disc (hereinafter, referred to as R and T directions). The four-sectioned light-receiving area E, F, G, and H receives +1st order light diffracted by the first diffraction area E″. The second through fifth light-receiving portions 55, 56, 57, and 58 include single light-receiving areas 55a, 56a, 57a, and 58a, respectively, which receive −1st order light diffracted by the second through fifth diffraction areas A″, B″, C″, and D″. The second through fifth light-receiving portions 55, 56, 57, and 58 also include bisectioned light-receiving areas 55b, 56b, 57b, and 58b, respectively, which receive +1st order light diffracted by the second through fifth diffraction areas A″, B″, C″, and D″.
Each of the bisectioned light-receiving areas 55b, 56b, 57b, and 58b is divided into inner and outer light-receiving areas in the T direction. The inner light-receiving area receives a central part of the light, and the outer light-receiving area receives a peripheral part of the light.
FIG. 3 illustrates the signal processing unit which detects a tracking error signal. In FIG. 3, reference characters E, F, G and H used to indicate the four-sectioned light-receiving area of the first light-receiving portion 53 also indicate detection signals of the four-sectioned light-receiving areas of the first light-receiving portion 53, respectively. Reference characters A, B, C, and D denote detection signals of the single light-receiving areas 55a, 56a, 57a, and 58a of the second through fifth light-receiving portions 55, 56, 57, and 58, respectively.
Referring to FIG. 3, the signal processing unit includes respective first, second, and third subtractors 71, 73, and 77. The first subtractor 71 detects a corrected far field (CFF) signal CFF from the detection signals A, B, C, and D of the single light-receiving areas 55a, 56a, 57a, and 58a of the second through fifth light-receiving portions 55, 56, 57, and 58 using a CFF tracking technique. The second subtractor 73 detects a push-pull (PP) signal PP from the detection signals E, F, G, and H of the four-sectioned light-receiving area of the first light-receiving portion 53 using a Push-pull technique. The third subtractor 77 subtracts between the signals CFF and PP and outputs a tracking error signal TESconventional. The signal processing unit further includes a gain adjuster 75 which amplifies the signal PP by a gain k′ and applies a result of the amplification to the third subtractor 77.
The signal CFF denotes a signal detected using a signal arithmetic process used in the Push-pull technique from signals (i.e., detection signals) detected from light-receiving areas of a photodetector which receive lights previously divided by a hologram.
When an objective lens (not shown) is shifted, the positions of the −1st order lights diffracted by the second through fifth diffraction areas A″, B″, C″, and D″ of the diffraction unit 30 and received by the single light-receiving areas 55a, 56a, 57a, and 58a of the second through fifth light-receiving portions 55, 56, 57, and 58 of the photodetector 50 are shifted. Consequently, the signal CFF has an offset of M and can be given by: CFF=M+N sin(wt) as shown in FIG. 3.
Since a cross-section of the +1st order light diffracted by the first diffraction area E″ and received by the four-sectioned light-receiving area E, F, G, and H of the first light-receiving portion 53 is enlarged in the R direction, the signal PP detected from the +1st order light is less sensitive to the shift of the objective lens. Accordingly, the signal PP is a DC signal having a magnitude of approximately m.
Hence, if the gain k′ of the gain adjuster 75 is set so that k′ m-M is 0, the third subtractor 77 outputs a tracking error signal TES that keeps a balance regardless of a shift of the objective lens.
In a conventional tracking method disclosed in the above U.S. Patent publication, the tracking error signal TES is obtained by subtracting between the signal CFF obtained from the detection signals A, B, C, and D and a signal obtained by multiplying the signal PP obtained from the detection signals E, F, G, and H by the gain k′. The gain k′ is obtained by dividing an inclination of the signal CFF generated upon a shift of the objective lens by an inclination of the signal PP generated upon the shift of the objective lens.
FIG. 4 illustrates offsets of the signal CFF, the signal PP, and the tracking error signal TESconventional upon a shift of the objective lens when the conventional tracking method disclosed in the above publication is used. As illustrated in FIG. 4, the offsets of the signals CFF and PP linearly increase in proportion to a shift amount of the objective lens, but the tracking error signal TES has no offset.
As a result, when the conventional tracking method disclosed in the above publication is used, a tracking error signal TES in which an offset generation is low even upon a shift of the objective lens can be detected.
However, when there is an initial offset due to an initial photodetector balance deviation, the offset is amplified. In other words, since the signal PP is detected using a typical push-pull technique, in which light reflected by a recording medium is divided according to a partition structure of a photodetector to detect a PP signal, the signal PP is affected by the initial photodetector balance deviation, and thus the initial offset due to the initial photodetector balance deviation is amplified.
FIGS. 5A and 5B are used to explain an influence of an initial photodetector balance offset upon a PP signal detected using the typical push-pull technique. FIG. 5A illustrates light received by a photodetector 9 when there is no initial photodetector balance offset. FIG. 5B illustrates the shift of light received by the photodetector 9 when there is an initial photodetector balance offset. In FIG. 5B, d denotes an initial photodetector balance deviation. The initial photodetector balance deviation denotes a distortion of a photodetector balance upon assembly of an optical pickup optical system. The initial photodetector balance offset denotes an offset generated due to the initial photodetector balance deviation.
An optical system illustrated in FIGS. 5A and 5B corresponds to a major part of an optical pickup that can detect a PP signal using a push-pull technique. In the optical system of FIGS. 5A and 5B, a hologram 5 and a photodetector 9 having two light-receiving areas 9a and 9b are included to divide light reflected by a recording medium 1 into two parts. The hologram 5 is installed between an objective lens 3 and a collimating lens (or a detection lens) 7.
As can be seen from a comparison of FIGS. 5A and 5B, when the typical push-pull technique is used to detect a PP signal and there is an initial photodetector balance offset (d), an offset is generated in the PP signal. Hence, even when the shift amount of the objective lens 3 is zero, the offset of the detected PP signal does not become zero.
Hence, if there exists an initial photodetector balance offset as illustrated in FIG. 6 upon the use of the conventional tracking method disclosed in the aforementioned publication, the PP signal has a push-pull offset, and accordingly, a tracking error signal TES has a large offset.
FIG. 6 illustrates offsets of a CFF signal, a PP signal, and a TES with respect to a shift amount of an objective lens when there is the initial photodetector balance offset and the conventional tracking method disclosed in the above publication is used. In FIG. 6, ba is the initial photodetector balance offset. As illustrated in FIG. 6, the offset of the PP signal is a sum of the initial photodetector balance offset and an offset generated due to the shift of the objective lens.
Also, even when an initial photodetector balance is mismatched, an offset due to the initial photodetector balance deviation is not generated in the CFF signal. In other words, an offset is generated in the CFF signal only when the objective lens is shifted.
However, when there exists an initial photodetector balance offset, the PP signal has an offset, and accordingly, a tracking error signal TES has a larger offset. The offset of the tracking error signal TES is a product of the offset of the PP signal and the gain k′.
As illustrated in FIG. 6, in the tracking error signal TES detected according to the conventional tracking method to remove an offset generated due to a shift of an objective lens, the offset due to the initial photodetector balance deviation is more amplified than that included in the PP signal.
In the above publication, light division by the diffraction unit 30 is not an order division but an area division, such that the offset amount of the PP signal may be changed by a transfer between reproduction/recording and a transfer between recorded/unrecorded areas.
Hence, when an offset due to an initial photodetector balance deviation exists in such a conventional optical pickup as described in the above publication, a tracking error signal has a large offset, and the offset of the tracking error signal varies depending on the transfer between reproduction and recording and the transfer between recorded and unrecorded areas. Further, the conventional optical pickup may record data on a recording medium while being detracked due to an offset of the tracking error signal.